In electrophotographic applications such as xerography, a charge retentive surface is electrostatically charged, and exposed to a light pattern of an original image to be reproduced, to selectively discharge the surface in accordance therewith. The resulting pattern of charged and discharged areas on that surface form an electrostatic charge pattern (an electrostatic latent image) conforming to the original image. The latent image is developed by contacting it with a finely divided electrostatically attractable powder referred to as "toner". Toner is held on the image areas by the electrostatic charge on the surface. Thus, a toner image is produced in conformity with a light image of the original being reproduced. The toner image may then be transferred to a substrate (e.g., paper), and the image affixed thereto to form a permanent record of the image to be reproduced. The process is well known, and is useful for light lens copying from an original, and printing applications from electronically generated or stored originals, where a charged surface may be discharged in a variety of ways.
It is common practice in electrophotography to use corona charging devices to provide electrostatic fields driving various machine operations. Thus, corona charging devices are used to deposit charge on the charge retentive surface prior to exposure to light, to implement toner transfer from the charge retentive surface to the substrate, to neutralize charge on the substrate for removal from the charge retentive surface, and to clean the charge retentive surface after toner has been transferred to the substrate. These corona charging devices normally incorporate at least one coronode held at a high voltage to generate ions or charging current to charge a surface closely adjacent to the device to a uniform voltage potential, and may contain screens and other auxiliary coronodes to regulate the charging current or control the uniformity of charge deposited. A common configuration for corotron corona charging devices is to provide a thin wire coronode tightly suspended between two insulating end blocks which support the coronode in charging position with respect to the photoreceptor and also serve to support connections to the high voltage source required to drive the coronode to corona producing conditions. Alternatively a pin array coronode may be provided, which substitutes an array of corona producing pin tips for the wire coronode, as shown for example in US-A4,725,732 to Lang et al. Scorotron corona charging devices have a similar structure, but are characterized by a conductive screen or grid interposed between the coronode and the photoreceptor surface, and biased to a voltage corresponding to the desire charge on the photoreceptor surface. The screen tends to share the corona current with the photoreceptor surface. As the voltage on the photoreceptor surface increases towards the voltage level of the screen, corona current flow to the screen is increased, until all the corona current flows to the screen and no further charging of the photoreceptor takes place. For this reason, scorotrons are particularly desirable for applying a uniform charge to the charge retentive surface preparatory to imagewise exposure to light.
In use, scorotron grids are commonly self-biased from corona current, by connecting the screen to a ground arrangement through current sink devices, such as discussed in US-A4,638,397 to Foley. In that particular example, a Zener diode and variable impedance device are arranged in series between the grid and ground and selected and set to maintain a selected voltage at the grid. US-A4,233,511 to Harada et al., and US-A4,603,964 to Swistak similarly disclose self-biasing scorotrons. Arrangements which adjust the bias applied to optimize the charging function are demonstrated in US-A4,618,249 to Minor and US-A4,638,397 to Foley.
In electrophotographic systems, it is commonly required to provide power supplies supplying a high voltage and low current to operate various devices within a machine. Examples of a devices requiring such power supplies are the developer bias arrangement or a closed loop electrostatic voltmeter (ESV) arrangement, typically used to measure photoreceptor voltage, and which may drive a feedback arrangement for controlling the voltage applied to the photoreceptor. In closed loop ESV's, a reference voltage is varied in accordance with the detected difference between this reference voltage and the photoreceptor voltage. This absolute reference voltage is then measured to determine the voltage on the photoreceptor. A significant cost in such devices is a high voltage power supply to drive the device, and a floating low voltage power supply to drive the feedback electronics, which usually requires a power supply with an oscillator-driven transformer to provide the bias voltage required. Such a circuit is a high cost item because of the inherent cost of transformers. Additionally transformers cannot be made on a low cost semiconductor device. In addition to the cost of such a device, the power supply also takes up space in a compact area. US-A4,714,978 to Coleman shows a power supply for an A.C. corotron which provides a feedback control of the power supply in accordance with variations in corona current. US-A4,433,298 to Palm describes a closed loop feedback arrangement with an ESV controlling various devices in an electrophotographic device. In the Xerox 3300 copier, the developer bias was driven from the corotron power supply through a very large, high power resistor to avoid the need for an extra power supply. All of the references cited herein and above are incorporated by reference herein for their teachings.